Studies on muscarinic acetylcholine receptors address the following questions: (1) How does ligand activation of the receptor influence the behavior of other membrane bound proteins? (2) What is the mechanism of activation and desensitization of the receptors of activation, and (3) What are the properties of the different forms of the receptor. The approach taken is the study of both ligand-binding and physical properties of muscarinic receptors, comparing particulate and solubilized forms of the receptors.